A supply pump actuating turbine, or SPAT, serves, in a power plant, for driving a supply pump which supplies feed water to a steam generator. A steam generator of this type is required, above all, in a steam turbine which operates with the steam of the steam generator.
Conventionally, a SPAT of this type comprises at least two jet sectors, with the aid of which a working gas can be introduced into the turbine, that is to say, to the SPAT. The working gas is expediently steam which is in any case available in a steam turbine in conjunction with a steam generator. Furthermore, the SPAT comprises a line system, with the aid of which the at least two jet sectors can be connected to at least two different sources of working gas. When the power plant is operating normally, the SPAT is supplied with bleed steam which is bled off at a suitable point on a steam turbine, preferably on a medium-pressure turbine. The bleeding point of the steam turbine at which the bleed steam is bled off forms in this case one source of working gas.
In specific operating states of the power plant, no bleed steam is available. So that the SPAT can nevertheless be operated, it is possible to use cold resuperheated steam for this purpose. In a steam turbine group which consists at least of a high-pressure turbine and of a medium-pressure turbine, the steam expanded in the high-pressure turbine is superheated in a steam generator or in a steam heater before it is supplied to the medium-pressure turbine. This superheating between the two turbines is designated as resuperheating. The cold resuperheated steam in this case corresponds to the expanded steam which comes from the high-pressure turbine and which is not yet superheated. A steam line which transports the steam from the high-pressure turbine to the steam heater forms in this case a further source of working gas.
Furthermore, the SPAT may be equipped with a valve arrangement which makes it possible to set the supply of the jet sectors with the working gas from at least one of the two sources, particularly in terms of the quantity of working gas supplied.
Under specific preconditions, it may be necessary to maintain a special operating state deviating from normal operation even for a relatively long period of time. It is consequently desirable that the SPAT has as high efficiency as possible both in normal operation with bleed steam and in special operation with cold resuperheated steam.